This application claims priority to European Patent Application EP 01870226.6 entitled xe2x80x9cAMBIPOLAR ORGANIC TRANSISTORxe2x80x9d and filed on Oct. 24, 2001. The disclosure of the above-described filed application is hereby incorporated by reference in its entirety.
1. Field of the Invention
The invention relates generally to microelectronics, and more particularly, to organic field-effect transistors having an organic active layer material.
2. Description of the Related Technology
Organic transistors, and more particularly thin film field-effect transistors, continue to become more attractive in the field of electronics because of their low-cost deposition technologies and steadily improving performance. An organic field-effect transistor consists of materials ranging from conductors and semiconductors, to insulators. A transistor is namely p-channel if the majority charge carriers in the channel are positive (holes), and n-channel when the majority charge carriers in the channel are negative (electrons). Complementary circuits, such as complementary metal oxide semiconductors (CMOS), consist of both n-channel and p-channel transistors. These complementary circuits can be particularly useful due to advantages such as low static power dissipation and simple circuit design.
For fabrication of organic complementary circuits, the organic semiconducting p-channel and n-channel materials should exhibit relatively high and comparable mobility values for holes as well as for electrons. Ambipolar operation has been observed in organic field-effect transistors based on ultra-pure pentacene single crystals (Schxc3x6n et al., Science, Vol. 287 (2000) p1022-1023). In the single crystals, which were grown by physical vapor transport in a stream of nitrogen, accumulation (p-type) and inversion (n-type) could be established.
However, low-cost solution-processable complementary circuits typically require the use of two different organic materials to obtain n-channel and p-channel transistors, since defects in these materials only allow unipolar charge transport.
There are several reasons why only a limited number of high performance n-channel organic semiconductors have been discovered so far, one of them is the fact that most organic materials tend to transport holes better than electrons and a large research effort is ongoing on electron transporters. An important difficulty in the field of organic transistors is finding a material which is not sensitive to oxygen, where oxygen is known to be an efficient trap for electrons.
Recent developments disclose organic transistor structures, comprising a mix of two active materials which enables p-channel and n-channel operations in the same device. An example of active materials exhibiting such properties are poly (3-hexylthiophene) (P3HT) and N,Nxe2x80x2-bis(2,5-di-tert-butylphenyl)-3,4,9,10-perylene dicarboximide (BPPC) as hole and electron transporters, respectively, where currents of both polarities can be injected from the source and drain contacts. Tada et al., Jpn. J. Appl. Phys., Vol. 36 (1997) Pt. 2, No. 6A, L718. Such transistors are n-channel under positive gate bias and p-channel under negative gate bias. Hole and electron mobilities of respectively less than 10xe2x88x926 cm2/(Vs) and less than 10xe2x88x927 cm2/(Vs) were reported for P3HT doped with a 25 mol % concentration of BPPC.
In Applied Physical Letters, Vol. 79, No. 2, 9, July 2001, Balberg describes a fullerene (C60)-polymer network composite structure showing a connectivity of a percolative C60 network and, additionally, a connectivity of the polymer network. In Balberg""s structure, the effect of the coupling phenomenon and the recombination kinetics depends on the molar fractions of both components.
In U.S. Pat. No. 5,629,530, Brown describes a solid state mixture of organic donor and acceptor molecules where difficulties are encountered in obtaining good balanced mobility values for holes and electrons transports in the organic donor/acceptor layer.
In U.S. Pat. No. 5,596,208, Dodabalapur describes organic field-effect transistors based on superposed evaporated organic n-type and p-type layers. In Dodabalapur""s transistors, evaporated C60 molecules are used as electron transporters in combination with Cr/Au bottom contacts. This technology leads to high production costs, which is a disadvantage for organic transistors as they are generally developed for low cost applications.
In 2000, Geens et. al (CP544, Electronic Properties of Novel Materialsxe2x80x94Molecular Structures, ed. H. Kuzmany et al., American Institute of Physics, pages 516-520) reported field effect mobility measurements of organic field-effect transistors using an active region consisting of a conjugated polymer poly (2-methoxy-5-(3xe2x80x2,7xe2x80x2-dimethyloctyloxy)-1,4-phenylene-vinylene (MDMO-PPV) blended with a soluble derivative of C60 (6,6)-phenyl C61-butyric acid methyl ester (PCBM) in ratios of 1:2, 1:3, 1:4 and 1:10 with gold source and drain electrode, p-conducting silicon as the gate electrode, and a silica insulating layer. However, Geens et al. did not report any transistor with ambipolar properties.
Brabec at al. (Synthetic Metals, 2000, Volume 121, pages 1517-1520) reported bulk heterojunction photovoltaic devices based on a spin-coated blend of MDVO-PPV and PCBM. Devices with power efficiencies higher than 2.5% under AM1.5 were reported.
In response to the deficiencies in the above described devices, a network of organic molecules providing high balanced mobility values for holes and electrons combined to single top contacts for low-cost large scale organic ambipolar transistor productions would be beneficial in the technology.
According to one aspect of the invention, an organic field-effect transistor comprises a source electrode, a drain electrode, wherein the source electrode and drain electrode are independent of one another located on a layered structure, an organic semiconductor layer forming a portion of the layered structure, an insulation layer underlying the organic semiconductor layer, and a gate electrode, underlying the insulation layer. The organic semiconductor layer comprises a channel between the source electrode and the drain electrode, and each of the source and drain electrodes comprises at least one of Au and (LiF and Al). The organic semiconductor layer further comprises hole and electron transporters, wherein the electron transporters comprise (6,6)-phenyl C61-butyric acid methyl ester (PCBM), and the hole transporters comprise at least one of poly (2-methoxy-5-(3xe2x80x2, 7xe2x80x2-dimethyloctyloxy)-1, 4-phenylene-vinylene)(OC1C10-PPV), and poly (3-hexylthiophene) (P3HT).
In an additional aspect of invention, the organic semiconductor layer comprises traces of an organic solvent, or more specifically, traces of at least one organic solvent selected from the group consisting of chlorobenzene, 1,2 dichlorobenzene, and xylene.
According to another aspect of the invention, a method of manufacturing an organic field effect transistor comprises depositing an insulating layer on a gate electrode, depositing an organic semiconductor layer on the insulating layer, and forming a source electrode and a drain electrode on the organic semiconductor layer, wherein the source electrode and the drain electrode are formed independently of one another. The organic semiconductor layer comprises a mixture of at least two components, wherein a first component comprises (6,6)-phenyl C61-butyric acid methyl ester, wherein a second component comprises at least one of poly(2-methoxy-5-(3xe2x80x2,7xe2x80x2-dimethyloctyloxy)-1,4-phenylene-vinylene)(OC1C10-PPV) and poly(3-hexylthiophene) (P3HT), and wherein both the source and drain electrodes comprise at least one of Au and (LiF and Al).
In yet another aspect of the invention, the method further comprises forming the gate electrode on a substrate. The insulating layer may be SiO2 layer, and forming the source and drain electrodes may comprise codeposition of Au and LiF in a weight ratio in the range of 1:10 to 10:1. In an additional aspect of the invention, the codeposition of Au and LiF can be followed by the deposition of an Al layer.
In yet another aspect of the invention, the organic semiconductor layer is deposited from a solution or dispersion in one or more organic solvents, or the organic semiconductor layer is deposited by spray-drying.